Phylum: Charophyta   Family: Desmidiaceae

Cosmarium exiguum W. Archer 1864

A characteristic species about twice as long as broad. It needs to be carefully compared with the recently described C. owencashlaense Kouwets 2025, but that species is on average slightly narrower giving a larger L/B ratio, apparently without overlap.
Cell dimensions: L: 25-27 µm, B: 12.4-13.3 µm, Is: 4-1-4.6 µm; Th: c.9-11.5 µm, L/B: 1.91-2.18.
West & West (1908: 63) specifically mention this species as being “Common in the Outer Hebrides”. Recent sampling would suggest it's still reasonably common; not surprising as there is ample suitable habitat. Its preferred habitat is acidic moorland pools and bogs, especially with Sphagnum.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.